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Oct 11, 2010, 07:41 AM
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Bisso's Avatar
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Build Log

6" across micro tricopter based on HK250 parts & MultiWii

Hi there,

Here's the build log of my tricopter.
I hope this can give some ideas to some, and we can discuss the strengh (if any ) or weakness of this design)

The main idea is to build something small and with only a few specificly built parts.
For these reasons, I decided to build from KH-250 (t-rex 250 clone) parts.
This has some advantages:
- easy build, no need of complex tools
- easy repair, readily available parts

First of all, I made a sketchup model of the tricopter.
I like to do such modelisation, as it's easy to draw "to scale" and assemble pieces lego-style, and you easily get an early overview of what it's going to look like
Another benefit is that you can export the faces and print it to actual size, eg. for cutting the main frame

Different frame size were designed:

(Note: I used a larger Turnigy motor model from the library and scaled it down to the size of the one I planned to use)

The "rudder" part went through several discussions and iterations:

I also made a small physics simulation to get a sense of how the rudder would react:

Google Sketchup - Sketchyphysics - Tricopter Rudder (2) (0 min 44 sec)

Then, it was time to build

I ordered the parts I needed (all from hobbyking):
---- Hardware
7x HK-250GT Metal Horizontal Fin Band
3x HK-250GT Metal Tail Boom Holder
3x HK-250GT Tail Boom
1x HK-250GT Metal Tail Servo Mount
1x Woven Carbon Fiber Sheet 300x100 (1.0MM Thick)
1x 8x3x3mm bearing (4pcs/set) (tail shaft bearings from t-rex 450)
1x T-rex 450 feathering shaft
---- Electronics and props
3x Turnigy 1811 Brushless Indoor Motor 1500kv
3x TURNIGY Plush 10amp 9gram Speed Controller
3x Prop saver for C-10 motor
1x GWS EP Propeller (DD-5030 127x76mm) (6pcs/set)
---- Battery
1x ZIPPY Flightmax 800mAh 2S1P 20C

---- Total: 110$

Additionaly I used 2 rudder joint balls and ball link from the t-rex 250
Note that Align's original horizontal stab holder is rond at the top, so it won't fit that well if you plan to add a flat motor support on top of it

Here it is, ready to build

It seems I had some luck at this point, as the distance between motor mount holes was exactly equal to the distance betweens the holes on the stab support, so there was no need for a carbon motor support

Printed the frame outline and went on with the carbon cutting (note to self: never build with carbon again )

Drilled holes and assembled boom holders and booms:

The rudder articulation went throught further modifications; I used a 450 size spindle shaft and glued (with red threadlock) the 4 bearings on it witha spacer in the middle, then I glued the external ring of the bearing to the inside wall of the boom. It requires a bit of patience and attention not to glue the whole bearing, but once it's glued, it won't move

Then I soldered and assembled the three motors and ESCs:

The tricopter spans about 20cm (6") motor to motor
AUW should be around 200gr.

I made a small harness to connect the 3 ESCs to the lipo, it's made a a small strip of metal bent ini circle around the 4 wires, so that it holds everything in place while you melt some solder into it

Here's a little test, 1 ESC plugged to the receiver, the other 2 with only signal plugged. Giving some input on each channel to test everything works OK:

Hk250 tricopter - motor test (0 min 32 sec)

For the rudder part, I use an additional fin holder with a ball joint screwed on its side, and servo holder just like on the original helicopter.
Here's the result:

Hk250 tricopter - rudder test (0 min 36 sec)


To be continued

Note: I have no plan for the landing gear yet, any suggestion is appreciated
Last edited by Bisso; Oct 12, 2010 at 03:49 PM.
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Oct 11, 2010, 07:42 AM
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Oct 11, 2010, 01:34 PM
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Loooks cool.

Do not forget to make room for the multiwii board.

Might want to try a 3s those motors like the power

That servo would not last me five flights of really aggressive flying .
Oct 12, 2010, 03:18 AM
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Originally Posted by witespy
Loooks cool.
Do not forget to make room for the multiwii board.
Might want to try a 3s those motors like the power
That servo would not last me five flights of really aggressive flying .

The board should sit on the top plate, I may add a kind of shield when it's finalized to protect it from the blades

I'm not sure 3S would be necessary. As you can see on the video, the tricopter flips over itself with only one motor. The three of them should produce enough lift to rocket it However I'll see that on the first few flight, and I have some 3s readily available from the t-rex 250

The servo on the picture is here only for illustration purpose. This will get replaced by a ~9gr servo

EDIT: I had an interesting discussion on another forum that shed some light on the 2s/3s alternative, most tricopters use 3s lipo with ~1000kv motors, that leads to 11100rpm; if I go with 2s lipo, with 1500kv motors, it gives me 11100rpm too. things may have been different if I had a 1000kv motor, as I'd only get 7400rpm from this combo
Last edited by Bisso; Oct 12, 2010 at 05:16 AM.
Oct 12, 2010, 03:44 AM
Registered User
very nice indeed.
Oct 12, 2010, 01:04 PM
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Bisso's Avatar
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Parts received, updated with a video of the rudder action
Oct 12, 2010, 01:40 PM
Z06 Tony's Avatar
How about the 250 LG. That should work nice and it is light. You can also get the generic CF LG. I think it says Tarot 250 on the pkg. I used to have a few sets. One thing I noticed with the HK CF sheets is you should sand the shine off where you are going to use velcro or it doesnt stick well and pulls right off.
Oct 13, 2010, 03:48 AM
la fabrique circulaire
terry74's Avatar
excelent Bisso
Oct 13, 2010, 04:50 AM
Registered User

Seems more like 6in radius than span though.

Looking forward to a flying vid.
Oct 13, 2010, 05:02 AM
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Bisso's Avatar
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Originally Posted by somnambulus

Seems more like 6in radius than span though.

Looking forward to a flying vid.
In fact it's 6" motor to motor
With 5" props, it's 7.5" edge to edge
Oct 13, 2010, 12:46 PM
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v22chap's Avatar
This is a cutie ,,, I also was thinking just use the 250 landing gear .
Oct 15, 2010, 08:23 AM
Suspended Account
sweet build so far bruv
Nov 14, 2010, 11:32 AM
Registered User
Hey, Nice setup, I did similar with using HK 250 GT parts, I did my tail a little differently, but, check it out. Basicaly, I used a basic block, with two Lightweight ceramic bearings Around the Tube. This way I was able to run my wires thru the tube, making a cleaner look.

I used a nother plate for the landing gear.

Nov 21, 2010, 07:03 AM
Father Francis, formerly Apach
Hey gang,

I have been checking back to this post for a bit now and am wondering, has there been any progress? I think it's a sweet little project and am hoping to see the end results and perhaps a few comments and video.

Nov 22, 2010, 08:29 AM
Father Francis, formerly Apach
Any updates Bisso?....

Or has this thread gone on vacation like so many others?......


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